1. Field of Invention
The present invention relates to a technique field of infrared focal plane arrays, and more particularly to a readout circuit for an uncooled infrared focal plane array.
2. Description of Related Arts
If a microbolometer infrared imaging system, as one kind of thermal infrared detector, is not compensated by special methods, a detection result thereof is associated with the substrate temperature. In practice, infrared detection results are expected to be only associated with the temperature of the object, and independent of other factors.
The thermo-electric cooler (TEC for short) utilized in the conventional uncooled microbolometer infrared imaging system compensates the substrate temperature. However, TEC has a certain volume and power consumption, which limits the application of uncooled infrared focal plane array detector. Therefore, TEC is attempted to be removed. However, in the microbolometer infrared imaging system without TEC, non-ideal effects such as extreme non-uniformity and non-linearity of the focal plane array will be caused by the change of the substrate temperature, thereby affecting a readout result.
Key technologies for solving the non-ideal effects of the uncooled infrared focal plane array detector without TEC are focused on: designing a readout circuit with a function of non-uniformity calibration and substrate temperature compensation. At the meantime, converting the analog signal to digital signal inside the chip and outputting the digital signal is an effective method for ensuring the quality of images.
The conventional readout circuit of the uncooled infrared focal plane array with analog-to-digital function obtains infrared radiation signals by constant voltage bias active microbolometers and reference microbolometers, wherein column level integrated reference microbolometers are added to the circuit for compensating the DC offset. The obtained infrared radiation signals are amplified by an integrator, and then are converted to digital signals by an analog-to-digital converter for being outputted. However, these output digital signals are substrate temperature dependent. To compensate the substrate temperature induced effects, various methods have been developed. The first method is based on the realization of additional correction of output signals in dependence on substrate temperature. The drawback of this method is the dramatic decrease of scene dynamic range. The second method is based on the preliminary heating and bias equalization (PHABEQ). The bias equalization (BEQ) methods are achieved by employing at least two digital-to-analog converters (DAC), one for BEQ and the other one is used to provide offset correction and substrate temperature compensation. As a result, the chip area and power consumption will be increased.